The present invention relates to a slow mode control circuit in a video tape recorder, and more particularly to a time difference slow mode control circuit for automatically compensating for reproduction control pulse pick-up error by generating a pseudo reproduction control signal.
In an operation of a tape recorder, a time difference slow mode means that a capstan motor repeats a still mode and a operating mode in turn by which displayed screens on a monitor look like a slow motion. At this time, the capstain motor should be accurately controlled when it repeats the still mode and the operating mode. Accordingly a reproduction control signal is used for an accurate control, because a microcomputer can decide a break time of the capstan motor by picking up the reproduction control signal which is recorded in a control track provided in a lower part of the tape. By reason of tape damage or something else, however, the reproduction control pulse may not be picked up, although the capstan motor is operating.
Operations of the microcomputer and a servo part now will be described with reference to FIG. 2. The servo part generates a head switching pulse with a period illustrated in FIG. 2A. Sensing a slow mode key input from a remote controller or a front panel, the microcomputer generates a step/slow signal as a first pulse mode in order to accelerate the capstan motor after a given time T1 is passed, the time T1 being measured from lowering of the head switching pulse, which is shown in FIG. 2B.
From an opening time of a screen reproduction by the capstan motor operation to a completion time of one frame of screen reproduction for a period of the head switching pulse, the step/slow signal maintains low state. At the completion of one screen reproduction, the servo part produces the reproduction control signal as shown in FIG. 2C. If the reproduction control signal is normally picked up, the microcomputer recognizes this control signal, and produces the step/slow signal of a second pulse mode when a given time T3 has passed after generation of the reproduction control signal. This makes the forwardly operating capstan motor stop, so that the time difference slow mode is achieved. The period T2 from the first pulse generating time to the second pulse generating time is the time required in order to move the tape. The break is applied to the capstan motor lest the period of the tape movement should be over the frame. Therefore, in order to drive the capstan motor in reverse direction for a given time, the microcomputer generates a capstan motor driving direction control signal as shown in FIG. 2E. As a result, the head can read the track exactly. As shown in FIG. 2D, the capstan motor driving signal supplied to the servo part from the microcomputer, keeps a first level L1 until the step/slow signal of the second pulse mode is generated from the driving of the tape, and drops to a second level L2 when the second pulse is generated. Here, the difference between the first level and the second level is needed in order to reduce the tape movement speed. That is, the tape movement speed is lowered at the second level L2, to turn over the tape movement direction.
In the case that the reproduction control pulse is not picked up by the microcomputer even when the capstan motor is operating, the microcomputer cannot provide the motor control signal to the the servo part, so that the servo part cannot exactly apply the break to the capstan motor, thus producing screen hunting or noise. In a conventional slow mode control circuit, however, a method for compensating for such a reproduction control pulse pick-up error is not provided.